Liquid absorbing acidic gases and use thereof in diacidification of gases

ABSTRACT

An absorbent liquid for improved absorption capacity for CO 2  in a gas due to the presence of an activator for CO 2  absorbent from the gas, the absorption liquid comprising a mixture of one or more tertiary alkanolamines and an activator of CO 2  absorption by the tertiary alkanolamine, which activator has the general formula Y--(CpH 2p )--NHZ wherein Y is a monovalent radical selected from the group consisting of piperazyl, piperidinyl, furyl, tetrahy-drothienyl and --OR 2  radicals, Z is an R 2  radical or hydrogen, R 2  is a monovalent hydrocarbon residue, and p is an integer from 1 to 6. Also a process for using such absorbent liquid for improved CO 2  absorption from a gas.

This patent application is a divisional application of allowedapplication Ser. No. 07/460,958, filed Jan. 18, 1990, for LiquidAbsorbing Acidic Gases and Use Thereof in Deacidification of Gases, nowU.S. Pat. No. 5,209,914, and a continuation in-part of copendingapplication Ser. No. 07/917,758, filed Jul. 21, 1992, pending, forLiquid Absorbent For Acidic Gases and Process of Deacidification of aGas, which are both incorporated herein by reference.

The invention relates to a liquid absorbing acidic gases containing atertiary alkanolamine component and an activator of CO₂ absorption. Italso relates to the application of the said absorbent liquid to thedeacidification of gases containing CO₂ and possibly other acidicgaseous compounds such as H₂ S or/and COS.

It is known that it is possible to rid gases of undesirable acidicgaseous compounds or acidic gases, especially CO₂, H₂ S and COS, whichthey contain by subjecting the said gases to scrubbing by means ofregenerable absorbent liquids consisting of organic solvents or ofaqueous solutions of organic solvents, which retain the acidic gaseouscompounds. On an industrial scale, there are chiefly two categories ofabsorbent liquids employed for scrubbing gas containing acidic gaseouscompounds, namely, on the one hand, absorbent liquids with physicalaction, such as sulpholane, N-methylpyrrolidone, polyethylene glycoldimethyl ether, methanol or else propylene carbonate, which absorbacidic gaseous compounds such as mentioned above without any chemicalreaction, but whose major disadvantage is that of simultaneouslyabsorbing large quantities of hydrocarbons and, on the other hand,absorbent liquids with predominant chemical action and especiallyaqueous solutions of alkanolamines such as monoethanolamine,diethanolamine, diglycolamine, diisopropanolamine, triethanolamine orelse methyldiethanolamine, which fix acidic gaseous compounds such asCO₂, H₂ S and COS by chemical reaction with formation of compoundscapable of being decomposed by heating, which allows the absorbentliquid to be regenerated. Primary alkanolamines such asmonoethanolamine, or secondary alkanolamines such as diethanolamine, aregenerally suitable for highly exhaustive removal of CO₂, but exhibit thedisadvantage of requiring a large expenditure of energy for theirregeneration.

With tertiary alkanolamines, especially methyl-diethanolamine andtriethanolamine, the consumption of energy needed for the regenerationis lower, but the treated gas resulting from the scrubbing operationusing the said tertiary alkanolamines still has CO₂ contents rangingfrom a few thousand ppm to a few per cent, depending on the case. Thisis due to the fact that tertiary alkanolamines do not react directlywith CO₂, the result being kinetics of absorption of CO₂ by theabsorbent liquid containing such tertiary alkanolamines which are muchslower than the kinetics of absorption of CO₂ by an absorbent liquidbased on primary or secondary alkanolamines, while the rate ofabsorption of H₂ S is substantially the same whatever the alkanolamineemployed in the absorbent liquid.

In order to draw maximum advantage of the saving of regeneration energywhich can be achieved when an absorbent liquid based on a tertiaryalkanolamine is employed for scrubbing gases containing acidic gaseousimpurities such as CO₂, H₂ S and COS, while obtaining a suitable removalof CO₂, it has been proposed to add to the said absorbent liquid a smallquantity of an activator which accelerates CO₂ absorption withoutmodifying the absorption capacity for other acidic gaseous compounds andespecially for H₂ S, or the energy saving achieved during theregeneration of the absorbent liquid.

Among the products proposed hitherto as activators of CO₂ absorption byabsorbent liquids based on tertiary alkanolamines are to be foundprimary alkanolamines such as monoethanolamine (EP-A-0,160,203),monomethylmonoethanolamine (U.S. Pat. No. 3,622,267) and piperazine(U.S. Pat. No. 4,336,233).

It has now been found that certain other amine compounds have been shownto be effective activators of CO₂ absorption by tertiary alkanolamines.

The invention proposes, therefore, a liquid absorbing acidic gaseouscompounds exhibiting an improved absorption capacity for CO₂, the saidabsorbent liquid containing a tertiary alkanolamine component made up ofone or more tertiary alkanolamines and an activator of CO₂ absorption bythe said tertiary alkanolamine component and characterized in that thesaid activator consists of at least one amino compound corresponding toone of the following formulae (I) ##STR1## with, in these formulae, Xdenoting a divalent radical chosen from C₂ -C₉ alkylene radicals and C₄-C₉ cycloalkylene radicals, the symbols R each denote independently ahydrogen atom or a C₁ -C₆ monovalent hydrocarbyl radical, optionallysubstituted by a functional group and, for example, by a hydroxyl group,with at least one of the symbols R consisting of a hydrogen atom, Ydenoting a monovalent radical chosen from the group formed by thepiperazyl, piperidinyl, furyl, tetrahydrofuryl, thienyl,tetrahydrothienyl and --OR₂ radicals, Z denoting an --R₂ radical or ahydrogen atom, R₁ denoting a C₂ -C₆ monovalent hydrocarbon radical, R₂being a hydrocarbon monovalent residue, optionally substituted by afunctional group and, for example, by a hydroxyl group, m and n, whichare identical or different, being integers taking the values of 2 to 6and preferably the values 2 or 3 and, p an integer ranging from 1 to 6.

When they do not denote hydrogen atoms, the symbols R appearing in theformulae (a) and (b) of the activators are in particular C₁ -C₆ alkyl orhydroxyalkyl radicals or phenyl radicals.

The amino compounds of formula (a) which may be chosen for forming theactivator according to the invention are advantageously those in thecase of which the symbols R in the formula which represents them arehydrogen atoms.

The preferred amino compounds of this type are the polyaminesrepresented by the following formula (e) ##STR2## in which n is aninteger ranging from 2 to 6 and preferably equal to 2 or 3 and p is aninteger ranging from 1 to 6 and preferably from 1 to 4.

Examples of such polyamines which may be especially mentioned aredipropylenetriamine (abbreviated to DPTA) of formula H₂ N--(CH₂)₃--NH--(CH₂)₃ --NH₂, diethylenetriamine (abbreviated to DETA) of formulaH₂ N--(CH₂)₂ --NH--(CH₂)₂ --NH₂ triethylenetetramine(abbreviated toTETA) of formula H₂ N--(CH₂)₂ --NH₂ (CH₂)₂ --NH₂ andtetraethylenepentamine (abbreviated to TEPA) corresponding to theformula H₂ N--(CH₂)₂ --NH₃ (CH₂)₂ --NH₂.

Advantageously, the amino compounds of formula (b) which may be chosento form the activator according to the invention are those in which, inthe said formula (b), the radical X is a C₂ -C₆ alkylene radical or acyclohexylene radical and each of the symbols R denotes a hydrogen atomor a C₁ -C₆ and preferably C₁ -C₄ alkyl or hydroxyalkyl radical,provided that at least one of the symbols R is a hydrogen atom.

By way of examples of such amino compounds of formula (b), there may bementioned especially aminoethylethanolamine (abbreviated to AEEA) offormula H₂ N--(CH₂)₂ --NH--(CH₂)₂ OH, hexamethylenediamine (abbreviatedto HMDA) of formula H₂ N--(CH₂)₆ --NH₂, dimethylaminopropylamine(abbreviated to DMAPA) of formula ##STR3## and 1,2-diaminocyclohexane(abbreviated to DACH) of formula ##STR4##

The amino compounds of formula (c) or (d) which may be employed asactivators according to the invention correspond in particular to theformulae Y₁ --(CH₂)_(q) --NH₂ and R₃ --NH--(CH₂)_(q) --OH, in which Y₁is a monovalent radical chosen from the radicals ##STR5## and C₁ -C₆alkoxy radicals, R₃ is a C₂ --C₆ alkyl radical, and q is an integertaking the values of 1 to 6.

Examples of such amino compounds are such as methoxypropylamine(abbreviated to MOPA), ethoxypropylamine, aminoethylpiperazine(abbreviated to AEPZ), aminopropylpiperazine, aminoethylpiperidine(abbreviated to AEPD), aminopropylpiperidine, furfurylamine (abbreviatedto FA) and ethylmonoethanolamine (abbreviated to EMEA).

The absorbent liquid according to the invention is generally in the formof an aqueous solution of the tertiary alkanolamine component and of theactivator. Where appropriate, the said aqueous solution may also containa minor quantity of one or more water-soluble organic solvents for theacidic gases, especially sulpholane, methanol or N-methylpyrrolidone.

The concentration of the tertiary alkanolamine component in the aqueousabsorbent liquid may be between 1N and 6N and preferably lies between2.5N and 5N.

The quantity of activator which is associated with the tertiaryalkanolamine component in the absorbent liquid according to theinvention can vary quite widely. The said quantity is advantageouslysuch that the ratio of the number of moles of activator to the totalnumber of moles of activator and of tertiary alkanolamine component isbetween 0.01 and 0.5 and preferably from 0.05 to 0.25.

The tertiary alkanolamines which can be employed for forming thetertiary alkanolamine component of the absorbent liquid may be chosenfrom the various tertiary alkanolamines which are soluble in water inthe concentrations employed. Examples of these tertiary alkanolaminesare such as N-methyldiethanolamine, triethanolamine,N-ethyldiethanolamine, 2-dimethylaminoethanol,2-dimethylamino-1-propanol, 3-dimethylamino-1-propanol,1-dimethylamino-2-propanol, N-methyl-N-ethyl-ethanolamine,2-diethylaminoethanol, 3-dimethylamino-1-butanol,3-dimethylamino-2-butanol, N-methyl-N-isopropylethanolamine,N-methyl-N-ethyl-3-amino-1-propanol, 4-dimethylamino-l-butanol,4-dimethylamino-2-butanol, 3-dimethylamino-2-methyl-l-propanol,1-dimethylamino-2-methyl-2-propanol, 2-dimethylamino-l-butanol and2-dimethylamino-2-methyl-l-propanol. The preferred tertiaryalkanolamines include methyldiethanolamine, triethanolamine,2-dimethylaminoethanol, 3-dimethylamino-1-propanol and1-diethylamino-2-propanol.

The absorbent liquid according to the invention can be employed forperforming the scrubbing of various gases containing CO₂ and possiblyother acidic gaseous compounds such as H₂ S and COS with a view tocarrying out a deacidification of these gases, that is to say a removalof the acidic gaseous compounds which they contain.

The gases to be treated containing CO₂ and possibly one or more otheracidic gaseous compounds such as H₂ S and COS may be synthesis gases,gases originating from coal gasification, coke oven gases, refinerygases or else natural gases, and their total content of acidic gaseouscompounds may range from a few tenths of a percent to some tens ofpercent by volume.

The implementation of the scrubbing of the gas containing CO₂ andpossibly the other acidic gaseous compounds to be removed, such as H₂ Sand COS, generally comprises an absorption stage during which the gas tobe treated and the absorbent liquid are brought into contact, preferablycountercurrentwise, in an absorption zone to produce a treated gas witha reduced content of acidic gaseous compounds, the said contentgenerally corresponding to that of the specifications imposed on thetreated gas, and an absorbent liquid charged with CO₂ and other possibleacidic gaseous compounds, and a regeneration stage, during which thesaid charged absorbent liquid is subjected to a regeneration treatmentto release the acidic gaseous compounds retained by the absorbent liquidand to produce, on the one hand, at least one acidic gaseous fractioncontaining the said released acidic gaseous compounds and, on the otherhand, at least one regenerated absorbent liquid which is recycledtowards the absorption zone.

The regeneration of the absorbent liquid charged with CO₂ and with otherpossible acidic gaseous compounds, especially H₂ S or/and COS, isadvantageously carried out by decompression, in one or more stages, ofat least a part of the said charged absorbent liquid, which is reflectedin a substantial saving in the energy to be employed for thisregeneration.

According to one embodiment of the regeneration, all of the chargedabsorbent liquid is decompressed in one or more stages to release mostof the CO₂ present in the said charged absorbent liquid, and thedecompressed absorbent liquid is then subjected to a complementaryregeneration by steam stripping, by direct or indirect heating of theabsorbent liquid, the absorbent liquid resulting from the complementaryregeneration being recycled into the absorption zone and especially intothe upper part of the said zone. In an alternative form of thisembodiment, only a part of the decompressed absorbent liquid issubjected to the complementary regeneration by stripping, the absorbentliquid resulting from the said complementary regeneration being, asindicated earlier, recycled into the upper part of the absorption zone,while the part of the decompressed absorbent liquid which is notsubjected to the complementary regeneration is recycled into theabsorption zone, at a point of the latter which is situated below thepoint of recycling of the absorbent liquid regenerated by stripping.

According to another embodiment of the regeneration, a fraction of thecharged absorbent liquid is decompressed to release most of the CO₂which it contains, while the remaining fraction of the charged absorbentliquid is subjected directly to a regeneration by steam stripping, bydirect or indirect heating of the said remaining fraction, the fractionof the absorbent liquid regenerated by stripping being recycled into theupper part of the absorption zone, while the decompressed fraction ofabsorbent liquid is recycled into the absorption zone below the point ofrecycling of the absorbent liquid regenerated by stripping.

If need be, the charged absorbent liquid leaving the absorption zone maybe subjected to a preliminary decompression to release nonacidic gasessuch as the hydrocarbons retained by the absorbent liquid, before theactual regeneration is carried out.

The absorption and regeneration stages outlined above can be implementedin any device which makes it possible to carry out the deacidificationof a gas by means of a regenerable absorbent liquid and in particular inthose of the said devices which make it possible to perform an at leastpartial regeneration of the charged absorbent liquid by decompressionand possibly to supplement this regeneration by a regeneration usingstripping. Devices similar to those which are shown diagrammatically inthe citations U.S. Pat. No. 3,622,267 and U.S. Pat. No. 4,336,233 areparticularly suitable.

The operating conditions for implementing the abovementioned absorptionand regeneration stages, especially temperature, pressure, gas flow rateand the flow rate of absorbent liquid, are those recommended for the gasdeacidification processes employing absorbent liquids based onalkanolamines. For example, the absorption stage during which the gas tobe treated, which contains CO₂ and possibly one or more acidic gaseouscompounds such as H₂ S and COS, is scrubbed using the absorbent liquid,can be implemented at temperatures of between 10° C. and 100° C. andmore particularly between 30° C. and 60° C. and at pressures of between1.5 and 100 bars absolute. Regeneration by decompression is also carriedout at the temperature at which the charged absorbent liquid to bedecompressed is present, the pressures reached after each decompressionbeing between the pressure of the charged absorbent liquid drawn offfrom the absorption zone and approximately 1.5 bars absolute anddecreasing from one decompression to the next when several successivedecompressions are performed. Regeneration by stripping is carried outin a conventional manner by subjecting the absorbent liquid to areboiling operation in a stripping zone maintained overhead at atemperature of approximately between 80° C. and 150° C. and at apressure below 5 bars and in most cases between 1.3 and 2.5 barsabsolute.

When the regeneration by decompression, in one or more stages, isfollowed by a complementary regeneration by stripping, the pressure ofthe decompressed absorbent liquid conveyed to the regeneration bystripping is chosen so as to be close to the overhead pressure of thestripping zone.

The invention is illustrated by the following examples, given withoutany limitation being implied.

EXAMPLE 1

Three series of tests were carried out, of CO₂ absorption usingabsorbent liquids according to the invention, consisting of aqueoussolutions of methyldiethanolamine (abbreviated to MDEA) and of anactivator of the polyamine type of formula (a) or, by way of comparison,using known absorbent liquids consisting of aqueous solutions of MDEAfree from activator.

In each test a gas containing CO₂ was scrubbed using the chosenabsorbent liquid by operating in a colunm equipped overhead with an exitfor the gases, in its upper part with an entry for the liquids, in itslower part with an entry for the gases and at the bottom with an exitfor the liquids, the internal space of the column included between thesaid entries for tile liquids and for the gases being provided with 12uniformly spaced perforated trays.

A gas containing 40% of CO₂ and 60% of methane by volume was injected ata flow rate of 600N l/h via the gas entry of the column, and the chosenabsorbent liquid was introduced at a flow rate of 3 l/h via the liquidentry of the said column. A CO₂ -depleted gas was discharged from thehead of the column and an absorbent liquid charged with CO₂ was drawnoff at the bottom of the said column.

The values of the absolute pressure and of the overhead temperature ofthe column were 2.2 bars and 50° C. respectively.

The gases entering and leaving the column were analysed by gas phasechromatography to determine their CO₂ content, and the quantity of CO₂absorbed by the absorbent liquid was deduced from these measurements.

The efficiency of the absorption of CO₂ by the absorbent liquidcontaining an activator was defined as a quantity called "relative CO₂absorption", which represents the ratio of the molar percentage of CO₂absorbed by the MDEA solution containing an activator to the molarpercentage of CO₂ absorbed by the MDEA solution without activator.

The operating conditions specific to each of the tests and the resultsobtained are collated in Table I below.

Inspection of the results which appear in the said Table shows that theabsorbent liquids according to the invention with MDEA base and anactivator of formula (a) exhibit an improved capacity for CO₂ absorptioncompared with absorbent liquids containing the same tertiaryalkanolamine, but without activator, this improvement in the efficiencyof CO₂ absorption being maintained and even increased when the absorbentliquids according to the invention contain a certain residual quantityof CO₂.

                  TABLE I                                                         ______________________________________                                        Composition of the aqueous absorbent liquid                                          Activator                                                                               Concen-    Residual                                                                             Relative                                   MDEA             tration    CO.sub.2                                                                             CO.sub.2                                   (moles/l)                                                                              Nature  (moles/l)  (g/l)  absorption                                 ______________________________________                                        4        --      --                1                                          3.5      DPTA    0.5               2.1                                        3.5      TETA    0.5               2.2                                        3.5      TEPA    0.5               2.3                                        3.5      DETA    0.5               2.3                                        4        --      --         20     1                                          3.5      DPTA    0.5        20     2.2                                        3.5      TETA    0.5        20     2.2                                        3.5      TEPA    0.5        20     2.4                                        3.5      DETA    0.5        20     2.4                                        4        --      --         40     1                                          3.5      DPTA    0.5        40     2.5                                        3.5      TETA    0.5        40     2.4                                        3.5      TEPA    0.5        40     2.7                                        3.5      DETA    0.5        40     2.7                                        ______________________________________                                    

EXAMPLE 2

Three series of tests were carried out, of CO₂ by absorbent liquidsaccording to the invention, consisting of aqueous solutions of methyldiethanolamine (abbreviated to MDEA) and of an activator of the diaminetype of formula (b) or, by way of comparison, by absorbent liquidsconsisting of aqueous solutions of MDEA free from activator.

In each test a gas containing CO₂ was scrubbed by means of the chosenabsorbent liquid by operating in a column similar to that described inExample 1, the internal space of the column included between the liquidentry and the gas entry being provided with 15 uniformly spacedperforated trays.

A gas containing, by volume, 40% of CO₂ and 60% of methane was injectedat a flow rate of 440N l/h through the gas entry of the column, and thechosen absorbent liquid was introduced at a flow rate of 3 l/h throughthe liquid entry of the said column. A CO₂ -depleted gas was dischargedoverhead from the column and an absorbent liquid charged with CO₂ wasdrawn off at the bottom of the said column.

The absolute pressure and the temperature at the head of the column hadvalues equal to 2.2 bars and 40° C. respectively.

The gases entering and leaving the column were analysed by gas phasechromatography to determine their CO₂ content and the quantity of CO₂absorbed by the absorbent liquid was deduced from these measurements.

The operating conditions specific to each of the tests and the resultsobtained are collated in Table II.

Inspection of the results which appear in Table II shows that theabsorbent liquids according to the invention based on MDEA and on anactivator of formula (b) exhibit an improved capacity for absorbing CO₂by comparison with the absorbent liquids containing the same tertiaryalkanolamine but free from activator, this improved absorption capacitybeing substantially maintained when the absorbent liquids according tothe invention contain a certain residual quantity of CO₂.

                  TABLE II                                                        ______________________________________                                        Composition of the aqueous absorbent liquid                                          Activator                                                                               Concen-    Residual                                                                             Relative                                   MDEA             tration    CO.sub.2                                                                             CO.sub.2                                   (moles/l)                                                                              Nature  (moles/l)  (g/l)  absorption                                 ______________________________________                                        4        --      --                1                                          3.5      AEEA    0.5               1.8                                        3.5      HMDA    0.5               1.7                                        4        --      --         22     1                                          3.5      AEEA    0.5        22     1.7                                        3.5      HMDA    0.5        22     1.5                                        4        --      --         42     1                                          3.5      AEEA    0.5        42     1.65                                       3.5      HMDA    0.5        42     1.45                                       ______________________________________                                    

EXAMPLE 3

Three series of tests were carried out, of CO₂ absorption by absorbentliquids according to the invention consisting of aqueous solutions ofMDEA and of an activator of the diamine type of formula (b) or, by wayof comparison, by absorbent liquids consisting of aqueous solutions ofMDEA free from activator.

In each test, a gas containing CO₂ was scrubbed by means of the chosenabsorbent liquid by operating in a column similar to that employed inExample 1, but equipped with 6 perforated trays.

A gas containing, by volume, 40% of CO₂ and 60% of methane was injectedat a flow rate of 600N l/h through the gas entry of the column, and thechosen absorbent liquid was introduced at a flow rate of 3l/h throughthe liquid entry of the said column. A CO₂ -depleted gas was dischargedat the head of the column and an absorbent liquid charged with CO₂ wasdrawn off at the bottom of the said column.

The absolute pressure and the temperature at the head of the column wereequal to 2.2 bars and 50° C. respectively. The gases leaving andentering the column were analysed by gas phase chromatography todetermine their CO₂ content and the quantity of CO₂ absorbed by theabsorbent liquid was deduced from these measurements.

The operating conditions specified to each of the tests and the resultsobtained are presented in Table III.

                  TABLE III                                                       ______________________________________                                        Composition of the aqueous absorbent liquid                                          Activator                                                                               Concen-    Residual                                                                             Relative                                   MDEA             tration    CO.sub.2                                                                             CO.sub.2                                   (moles/l)                                                                              Nature  (moles/l)  (g/l)  absorption                                 ______________________________________                                        4        --      --                1                                          3.5      DMAPA   0.5               2                                          3.5      DACH    0.5               2.1                                        4        --      --         15     1                                          3.5      DMAPA   0.5        15     1.9                                        3.5      DACH    0.5        15     2.1                                        4        --      --         30     1                                          3.5      DMAPA   0.5        30     1.9                                        3.5      DACH    0.5        30     2                                          ______________________________________                                    

Inspection of the results in Table III shows again the improved capacityfor absorbing CO₂ exhibited by the absorbent liquids according to theinvention based on MDEA and on an activator of formula (b) by comparisonwith the control absorbent liquids containing the same tertiaryalkanolamine but free from activator. In addition, this improved CO₂absorption capacity is substantially maintained when the absorbentliquids according to the invention contain a certain residual quantityof CO₂.

EXAMPLE 4

Three series of tests were carried out, of CO₂ absorption by absorbentliquids according to the invention, consisting of aqueous solutions ofmethyl diethanolamine (abbreviated to MDEA) and of an activator, namelyEMEA (formula (d)) or FA (formula (c)), or, by way of comparison, byabsorbent liquids consisting of aqueous solutions of MDEA free fromactivator.

In each test a gas containing CO₂ was scrubbed by means of the chosenabsorbent liquid by operating in a column similar to that employed inExample 1, but containing 9 uniformly spaced perforated trays.

A gas containing, by volume, 40% of CO₂ and 60% of methane was injected,at a flow rate of 440N l/h through the gas entry of the column, and thechosen absorbent liquid was introduced at a flow rate of 3l/h throughthe liquid entry of the said column. A CO₂ -depleted gas was dischargedat the head of the column and an absorbent liquid charged with CO₂ wasdrawn off at the bottom of the said column.

The absolute pressure and the temperature at the head of the column hadvalues equal to 2.2 bars and 40° C. respectively.

The gases entering and leaving the column were analysed by gas phasechromatography to determine their CO₂ content and the quantity of CO₂absorbed by the absorbent liquid was deduced from these measurements.

The operating conditions specific to each of the tests and the resultsobtained are collated in Table IV.

Inspection of the results which appear in Table IV shows that theabsorbent liquids according to the invention based on MDEA and on anactivator of formula (c) or (d) exhibit an improved capacity forabsorbing CO₂ by comparison with the absorbent liquids containing thesame tertiary alkanolamine, but free from activator, this improvedabsorption capacity being substantially maintained when the absorbentliquids according to the invention contain a certain residual quantityof CO₂.

                  TABLE IV                                                        ______________________________________                                        Composition of the aqueous absorbent liquid                                          Activator                                                                               Concen-    Residual                                                                             Relative                                   MDEA             tration    CO.sub.2                                                                             CO.sub.2                                   (moles/l)                                                                              Nature  (moles/l)  (g/l)  absorption                                 ______________________________________                                        4        --      --                1                                          3.5      EMEA    0.5               1.95                                       3.5      FA      0.5               1.85                                       4        --      --         20     1                                          3.5      EMEA    0.5        20     1.90                                       3.5      FA      0.5        20     1.75                                       4        --      --         40     1                                          3.5      EMEA    0.5        40     1.85                                       3.5      FA      0.5        40     1.70                                       ______________________________________                                    

EXAMPLE 5

Three series of tests were carried out, of CO₂ absorption by absorbentliquids according to the invention, consisting of aqueous solutions ofMDEA and of an activator of formula (c) or, by way of comparison, byabsorbent liquids consisting of aqueous solutions of MDEA free fromactivator.

In each test a gas containing CO₂ was scrubbed by means of the chosenabsorbent liquid by operating in a column similar to that employed inExample 1, but equipped with 6 perforated trays.

A gas containing, by volume, 40% of CO₂ and 60% of methane was injected,at a flow rate of 600N l/h, through the gas entry of the column, and thechosen absorbent liquid was introduced at a flow rate of 3 l/h throughthe liquid entry of the said column. A CO₂ -depleted gas was dischargedat the head of the column and an absorbent liquid charged with CO₂ wasdrawn off at the bottom of the said column.

The absolute pressure and the temperature at the head of the column wereequal to 2.2 bars and 50° C. respectively.

The gases entering and leaving the column were analysed by gas phasechromatography to determine their CO₂ content and the quantity of CO₂absorbed by the absorbent liquid was deduced from these measurements.

The operating conditions specific to each of the tests and the resultsobtained are shown in Table V.

                  TABLE V                                                         ______________________________________                                        Composition of the aqueous absorbent liquid                                          Activator                                                                               Concen-    Residual                                                                             Relative                                   MDEA             tration    CO.sub.2                                                                             CO.sub.2                                   (moles/l)                                                                              Nature  (moles/l)  (g/l)  absorption                                 ______________________________________                                        4        --      --                1                                          3.5      MOPA    0.5               1.95                                       3.5      AEPD    0.5               1.9                                        4        --      --         22     1                                          3.5      MPOA    0.5        22     1.85                                       3.5      AEPD    0.5        22     1.8                                        4        --      --         42     1                                          3.5      MOPA    0.5        42     1.8                                        3.5      AEPD    0.5        42     1.7                                        ______________________________________                                    

Inspection of the results which appear in Table V again shows theimproved capacity for absorbing CO₂ exhibited by the absorbent liquidsaccording to the invention based on MDEA and on an activator of formula(c) when compared with that possessed by the control absorbent liquidscontaining the same tertiary alkanolamine but free from activator. Inaddition, it appears that this improved absorption capacity for CO₂ issubstantially maintained when the absorbent liquids according to theinvention contain a certain residual quantity of CO₂.

EXAMPLE 6

Three series of tests were carried out, of CO₂ absorption by absorbentliquids according to the invention, consisting of aqueous solutions ofMDEA and of an activator consisting of aminoethylpiperazine (compound offormula (c)) or, by way of comparison, by absorbent liquids consistingof aqueous solutions of MDEA, free from activator.

In each test a gas containing CO₂ was scrubbed by means of the chosenabsorbent liquid by operating in a column similar to that employed inExample 1, but equipped with 12 perforated trays.

A gas containing, by volume, 40% of CO₂ and 60% of methane was injected,at a flow rate of 440N l/h, through the gas entry of the column, and thechosen absorbent liquid was introduced at a flow rate of 3l/h throughthe liquid entry of the said column. A CO₂ -depleted gas was dischargedat the head of the column and an absorbent liquid charged with CO₂ wasdrawn off at the bottom of the said column.

The absolute pressure and the temperature at the head of the column wereequal to 2.2 bars and 40° C. respectively. The gases entering andleaving the column were analysed by gas phase chromatography todetermine their CO₂ content and the quantity of CO₂ absorbed by theabsorbent liquid was deduced from these measurements.

The operating conditions specific to each of the tests and the resultsobtained are shown in Table VI.

                  TABLE VI                                                        ______________________________________                                        Composition of the aqueous absorbent liquid                                          Activator                                                                               Concen-    Residual                                                                             Relative                                   MDEA             tration    CO.sub.2                                                                             CO.sub.2                                   (moles/l)                                                                              Nature  (moles/l)  (g/l)  absorption                                 ______________________________________                                        4        --      --                1                                          3.5      AEPZ    0.5               2.2                                        4        --      --         15     1                                          3.5      AEPZ    0.5        15     2.1                                        4        --      --         30     1                                          3.5      AEPZ    0.5        30     2.1                                        ______________________________________                                    

Inspection of the results which appear in Table VI again shows theimproved capacity for absorbing CO₂ exhibited by the absorbent liquidsaccording to the invention based on MDEA and on an activator of formula(c) when compared with that possessed by the control absorbent liquidscontaining the same tertiary alkanolamine but free rom activator. Inaddition, it appears that this improved absorption capacity for CO₂ issubstantially maintained when the absorbent liquids according to theinvention contain a certain residual quantity of CO₂.

We claim:
 1. A process for improved CO₂ absorption from a gas by anabsorbent liquid which has an improved absorption capacity for CO₂ whichprocess comprises contacting said gas and the absorbent liquid in anabsorption zone having an upper and lower part, thereby obtaining atreated gas with a reduced content of CO₂ and the absorbent liquid withincreased CO₂ content, and regenerating the absorbent liquid which hasan increased CO₂ content, thereby releasing the CO₂ and producing atleast one gaseous fraction containing the released CO₂ and at least oneportion of the regenerated absorbent liquid and recycling saidregenerated absorbent liquid to the absorption zone, the absorbentliquid being a mixture of one or more tertiary alkanolamines and anactivator of improved CO₂ absorption by said tertiary alkanolamines, theactivator having the general formula

    Y--(C.sub.p H.sub.2p)--NHZ

wherein Y is a monovalent radical selected from the group consisting ofpiperazyl, piperidinyl, furyl, tetrahydrofuryl, thienyl,tetrahydrothienyl and --OR₂ radicals, Z is an --R₂ radical or hydrogen,R₂ is a monovalent hydrocarbon residue, and p is an integer from 1 to 6.2. The process of claim 1 wherein Y is a monovalent radical selectedfrom the group consisting of ##STR6## and C₁ -C₆ alkoxy radicals and Zis hydrogen.
 3. The process of claim 1 wherein the activator is selectedfrom the group consisting of methoxypropylamine, ethoxypropylamine,aminoethylpiperazine, aminopropylpiperazine, aminoethylpiperidine,aminopropylpiperidine and furfurylamine.
 4. The process of claim 1wherein in the formula for the activator, R₂ is substituted by afunctional group.
 5. The process of claim 4 wherein the functional groupis hydroxyl or phenyl.
 6. The process of claim 1 wherein the activatoris present in an activating amount.
 7. The process of claim 1 whereinthe gas is subjected to a treatment comprising an absorption stage and aregeneration stage during which the charged absorbent liquid issubjected to a regeneration treatment to release the CO₂ which it hasretained and to produce, on the one hand, at least one acidic gaseousfraction containing the released CO₂ and, on the other hand, at leastone regenerated absorbent liquid which is recycled towards theabsorption zone.
 8. The process of claim 7 wherein the regeneration iscarried out by decompression in one or more stages of at least a part ofthe charged absorbent liquid.
 9. The process of claim 1 wherein theregeneration of the charged absorbent liquid is carried out bysubjecting all of the charged absorbent liquid to decompression in oneor more stages to release most of the CO₂ present in the said chargedabsorbent liquid, then subjecting the decompressed absorbent liquid to acomplementary regeneration by steam-stripping by direct or indirectheating of the absorbent liquid, the absorbent liquid resulting from thecomplementary regeneration being recycled into the absorption zone. 10.The process of claim 1 wherein a part of the decompressed absorbentliquid is subjected to a complementary regeneration by stripping, theabsorbent liquid resulting from the complementary regeneration beingrecycled into the upper part of the absorption zone while the part ofthe decompressed absorbent liquid which is not subjected to thecomplementary regeneration is being recycled into the absorbent zonebelow the absorbent liquid regenerated by stripping.
 11. The process ofclaim 1 wherein the regeneration of the charged absorbent liquid iscarried by subjecting a fraction of the charged absorbent liquid todecompression in one or more stages to release most of the CO₂ which itcontains while the remaining fraction of the charged absorbent liquid issubjected directly to a regeneration by steam-stripping, by direct orindirect heating of the absorbent liquid of the remaining fraction, thefraction of the absorbent liquid regenerated by stripping is beingrecycled into the upper part of the absorption zone while thedecompressed fraction of absorbent liquid is being recycled into theabsorption zone below the absorbent liquid regenerated by stripping. 12.The process of claim 1 wherein the CO₂ -containing gas to be treatedalso comprises other acidic gas including H₂ S and/or COS.
 13. Theprocess of claim 7 wherein the decompressed absorbent liquid issubjected to a complementary regeneration step which comprisessteam-stripping the absorbent liquid by direct or indirect heating. 14.An absorbent liquid for improved absorption capacity for CO₂ in a gasdue the presence of an activator for CO₂ absorbent from said gas, theabsorption liquid comprising a mixture of one or more tertiaryalkanolamines and an activator of CO₂ absorption by said tertiaryalkanolamine, which activator has the general formula

    Y--(C.sub.p H.sub.2p)--NHZ

wherein Y is a monovalent radical selected from the group consisting ofpiperazyl, piperidinyl, furyl, tetrahydrofuryl, thienyl,tetrahydrothienyl and --OR₂ radicals, Z is an --R₂ radical or hydrogen,R₂ is a monovalent hydrocarbon residue, and p is an integer from 1 to 6.15. The absorbent liquid of claim 14 wherein in the formula for theactivator, R₂ is substituted by a functional group.
 16. The absorbentliquid of claim 15 wherein the functional group is hydroxyl or phenyl.17. The absorbent liquid of claim 15 wherein the activator is present inan activating amount.
 18. The absorbent liquid of claim 17 wherein theabsorbent liquid is an aqueous solution of the tertiary alkanolamine andthe activator.
 19. The absorbent liquid of claim 18 wherein theabsorbent liquid contains a minor quantity of at least one water-solubleorganic solvent for the CO₂, selected from the group consisting ofsulpholane, methanol and N-methylpyrrolidone.
 20. The absorbent liquidof claim 18 wherein the concentration of the tertiary alkanolamine inthe aqueous solution is between 1N and 6N.
 21. The absorbent liquid ofclaim 18 wherein the quantity of activator is such that the ratio of thenumber of moles of activator to the total number of moles of activatorand of tertiary alkanolamine is between 0.01 and 0.5.
 22. The absorbentliquid of claim 18 wherein the tertiary alkanolamine is selected fromthe group consisting of at least one of the tertiary alkanolaminesN-methyldiethanolamine, triethanolamine, 2-dimethylaminoethanol,3-dimethylamino-1-propanol and 1-diethylamino-2-propanol.
 23. Theabsorbent liquid of claim 18 wherein the activating amount of theactivator in the absorbent liquid is between 0.05 mole and about 2 molesper liter.
 24. The absorbent liquid of claim 23 wherein the activatingamount is from about 0.1 mole to about 1 mole per liter.
 25. Theabsorbent liquid of claim 20 wherein the concentration of the tertiaryalkanolamine is between 2.5N and 5N.
 26. The absorbent liquid of claim21 wherein the ratio is between 0.05 and 0.25.